You don't say how insulated the building is? I have installed several Empire direct vent heaters, and some Ecotherm units. They all use a twin tube vent, with the inner tube being the flue, the outer tube bringing in the combustion air. They seem to work well, and are pretty safe, in that you aren't combusting shop air. The Empire units had fans built in. For your building, you might want to look at a hanging space heater- they are also available with the outside air feed, I think. I presently heat a 32' x 24' x 8.5' shop with a 30,000 btu unit, no fan, but the building is pretty well insulated, and I aim for 60 degrees.
It takes a long time to bring up the heat (outside temps in the 20's, for example) so I try to keep it at 45 or so all the time. Insulation is the key... even some rigid foam boards under the slab perimeter.
